1. Field
The present disclosure relates generally to light to current converter devices (e.g., solar cells), and, more particularly, to back contact solar cells.
2. Related Art
In recent years, interest in new forms of renewable energy has increased due to problems associated with conventional energy sources. For example, rising oil prices, global warming, exhaustion of fossil fuel energy, nuclear waste disposal, locating construction sites for new power plants, and the like, have caused interest in photovoltaic devices or solar cells, which are pollution-free energy sources, to grow. As a result, research and development in this field has actively progressed.
A solar cell, which is an apparatus that converts light energy into electrical energy using the photovoltaic effect, may fall into any one of a number of different cell types, such as silicon solar cells, thin film solar cells, dye-sensitized solar cells, and the like. Silicon solar cells occupy the largest portion of current markets due to its high conversion efficiency. In standard-structured solar cells, positive and negative contacts are located on opposite sides of the solar cell. Therefore, shadow loss on the front or illuminated surface by the corresponding contacts limits the light to current conversion efficiency.
Some alternative solar cells have been developed to solve the above problem, one of which is called a “back contact solar cell.” In a back contact solar cell, both ohmic contacts (positive and negative contacts) are placed on the back or non-illuminated surface of the solar cell. In this way, shadow loss can be reduced or avoided.
One conventional way to fabricate back contact solar cells is to place the carrier collecting junction formed between oppositely doped semiconductor regions close to the back surface of the cell, rather than the front surface of the cell. This type of back contact solar cell is called a “back junction cell,” and is described in “1127.5-Percent Silicon Concentrator Solar Cell” (R. A. Sinton, Y. Kwark, J. Y. Gan, R. M. Swanson, IEEE Electron Device Letters, Vol. ED-7. No. 10 Oct. 1986). However, since the majority of photons are absorbed close to the front surface of the back junction cell, the carriers generated in these regions must diffuse through the entire base region of the cell before reaching the carrier collecting junction located near the back surface. Thus, these types of solar cells require high quality wafers having minority carrier diffusion lengths longer than the wafer thickness as well as very high minority carrier lifetimes.
Another way to fabricate back contact solar cells is to place both external contacts for the oppositely doped regions on the back surface of the solar cell and to place the collecting junction close to the front surface of the solar cell. In these devices, the collection current from the front surface is lead through openings, through-holes, or vias, which extend through the entire wafer to the back surface. Using this structure, shading losses caused by the front metallization may be reduced. The document WO 1998/054763 (EP0985233B1) describes such a structure, referred to herein as “Metal Wrap Through (MWT).” Furthermore, additional patents and patent applications, such as, WO2010126346, JP2010080576, JP2010080578, US20100276772, US20090188550, US20090178707, KR1020100098993, and DE102008033632, describe additions to the MWT structure. However, the described structures generally include a double junction with the emitter located on the front of the cell, back of the cell, and inside walls of the via holes. To illustrate, FIG. 1 shows a cross-sectional view of a P-N junction having an emitter 2 that covers the front surface, the full inner surface of the via hole 3, and the adjacent back side to the via hole of the substrate 1. To generate this type of P-N junction, double sided diffusion is needed, causing throughput loss during manufacturing. Additionally, back contact isolation using a laser is required to eliminate the short circuit that would otherwise occur between the backside emitter and the back contact. The use of the laser increases the breakage ratio of the solar cell, increases the production costs, and causes damage in the crystalline material leading to more recombination of charge carriers in the area around the laser groove.
To omit the back contact isolation step and reduce the excessive shunt, such as that occurring in the via and under the back emitter bus bars, the back side emitter can be removed and a dielectric layer can be positioned to cover the via and the adjacent back side of the solar cell, as disclosed in, for example, patents and patent applications: US20100319766, US20100258177, EP2068369, WO2009071561, CN101889349, US20110005582, US20090084437. Among these, some describe the emitter as being located only on the front surface, while others describe the emitter as being located on both the front surface and inner via holes. In general, these MWT structures involve the additional step of dielectric layer deposition, and other steps to remove the dielectric layer where it is not need. To illustrate, FIG. 2 shows a P-N junction having an emitter 2 that covers the front surface of the substrate 1 and a dielectric layer 12 that covers the full inner surface of the via hole 3. Additionally, FIG. 3 shows a P-N junction having an emitter 2 that covers the front and full inner surface of the substrate 1 and a dielectric layer 12 covering the full inner surface of the via hole 3.
Thus, efficient light to current converter devices and processes for making the same are desired.